Without limiting the scope of the present invention, its background will be described in relation to a high mechanical strength sorbent product, system, and method for controlling multiple pollutants from process gas, as an example.
The United States continually strives for cleaner energy production to decrease the detrimental impacts to our environment. For coal-fired power plants in particular this is a challenge that involves implementation of various pollution control devices. Over the years, as regulations continue to tighten and include more contaminants of concern, novel technologies that can remove multiple pollutants simultaneously have become popularized.
Activated carbon is a well-known material commonly used for treatment of a multitude of pollutants from gas and liquid streams. Activated carbons continue to find new treatment applications across many industries. In coal-fired power application, activated carbons have most recently been used to remove mercury from flue gas streams for protection of the environment. In other process gases, activated carbons have been demonstrated to remove sulfur dioxide (SO2) and volatile organic compounds (VOCs). Typically these applications involve using a specific activated carbon that has properties that are optimized for the particular pollutant. Certain characteristics that are ideal for one contaminant are not so for another. Therefore, the current state of art to control for various pollutants from process gas or flue gas is comprised of different targeted pollution control technologies to remove each pollutant individually.
For instance, using flue gas generated from coal-fired power production, nitrogen oxides (NOx), sulfur oxides (SOx), and mercury are regulated pollutants that require specific pollution control equipment to remove them to permissible limits. A specific technology is applied for each pollutant removal. First, selective catalytic reactors (SCR) usually precede other technologies with the aim of accomplishing denitrification. The options for desulfurization include dry and wet scrubbing. Mercury is effectively controlled with injection of powdered sorbents subsequently removed by particulate control devices. Multi-pollutant control is much desired in an era of increasingly stringent environmental regulations.
Less known and applied, shaped activated carbon pellets have been installed in a fixed bed configuration to provide multipollutant control (to remove sulfur dioxide, sulfur trioxide, mercury, particulate matter and nitrogen oxides from flue gas in a ReACT™ System). Current techniques to produce shaped activated carbon pellets for this application involve several production steps and high temperature treatments that produce excessive emissions. U.S. Pat. No. 5,840,651 describes a method for the production of an activated coke with high activity for desulfurization and denitrification. The method of such prior art is comprised of blending coals with different caking properties; forming a mixture; oxidizing followed by carbonizing and followed by further oxidation. These oxidizing and carbonizing steps are performed at elevated temperatures to transform the base material with inherently low specific surface area and low activity into a sorbent material suitable for desulfurization and denitrification. Several formulations include organic binders such as phenolic resins (U.S. Pat. No. 5,736,485) but these also require high temperature (i.e. energy intensive) processing and emit hazardous air pollutants during production.
Other production methods for shaped activated carbon pellets have been developed using clay binders, which circumvent many emission concerns but still require high temperature processing. Where high strength of the shaped sorbent is required, adsorption capacity is typically lost. For example, when clay is used as a binder, increasing the clay fraction from 5 to 30% significantly increases hardness. However, adsorption of butane, as an example, steadily decreases with the increased fraction of clay (U.S. Pat. No. 5,488,021).
Lower temperature production methods thus developed from the prior art insufficiency use cellulosic binder materials (U.S. Patent Application Publication No. 2003/0022787). However, these materials do not exhibit stability in high temperature applications and therefore would not be suitable for use in many process gas applications. There remains a need to produce shaped activated carbon bodies that have improved strength under a multitude of operating conditions with high adsorption capacity and reactivity produced at a competitive cost.